1. Field of the Invention
The present invention relates to hydraulic circuits for controlling variable hydraulic members, and particularly pressure-flow compensated variable displacement pumps in power-on-demand circuits.
2. Description of the Related Art
Pressure-flow compensated variable displacement pumps often are used in power-on-demand hydraulic circuits to reduce the energy consumption of the hydraulic system by limiting the output of the pump to the load requirements of the system. A typical use for such a system is in a work vehicle, e.g., agricultural or industrial equipment. In such an application, the output of the pump is throttled back to a minimum level when there is no load demand on the hydraulic system. As the load increases, the output of the pump is increased.
U.S. Pat. No. 4,293,284 (Carlson et al.) teaches a circuit for controlling a variable displacement pump in such a fashion. The output of Carlson's pump 12 increases as the pressure at the pump control piston 22 decreases, and vice-versa. Since the pressure vs. flow characteristics controlling the output of the pump vary depending on the load, a pressure-flow compensating valve 28 is provided to adjust the pressure applied to the control piston 22. The two-position valve 28 is biased towards a first position connecting the control piston 22 to sump and a second position connecting the control piston 22 to the outlet of the pump. A load sense line 46 conveys a pressure representative of the load demand on the pump to one end of the valve 28, moving it towards the first position, while the pressure at the pump outlet is applied to the other side of the valve. With this arrangement, the pressure output of the pump will keep increasing until it exceeds the load demand on the pump by the amount necessary to overcome the spring biasing valve 28.
The Carlton et al. arrangement has problems due to lag times. The load demand pressure takes a small but significant amount of time to travel through the load sense line 46 back to the pressure-flow compensating valve 28. However, due to the normal positioning of these valves immediately adjacent to the pump, the pump outlet pressure is applied to the valve almost immediately. As a result, the pump typically will overshoot the desired pressure, in both directions, that is, when the pressure should be increasing, it will increase too much and when it should be decreasing, it will decrease too much. As a result, the pump outlet pressure oscillates around the desired pressure rather than promptly stabilizing at that pressure.
U.S. Pat. No. Re. 29,673 (Van Gerpen) teaches a more complicated load sense circuit in which the pressure is conditioned before reaching the control piston 32, though the pressure remains representative of the load demand on the pump. In addition to this conditioning, the Van Gerpen reference differs from the Carlson et al. reference in providing orifice 56 connecting the pump control piston 32 to sump. The orifice 56 assures a minimum pressure differential between the sump and the control piston whenever any pressure is provided through the line 54 from the load sense circuitry. In Carlson et al.'s circuit, there can be flow to and from the control piston 22, but not past it. In contrast, in Van Gerpen, there is flow past the control piston 32 through the orifice 56.